1. Field of the Invention
The present invention relates to an optical transmission apparatus, an optical transmission system, and an optical transmission control method which are used for transmitting a wavelength division multiplexed optical signal, and more particularly, to an optical transmission apparatus, an optical transmission system including the same, and an optical transmission control method thereof having a function of adding or dropping an optical signal of an arbitrary wavelength.
2. Description of the Related Art
In recent years, along with developments in a photonic network, an optical add/drop multiplexing (OADM) technology is being applied to a wavelength division multiplexing system (WDM system). The OADM technology is particularly essential in a metropolitan area and access method network which connects a user side and a main network. This is because an optical signal of an arbitrary wavelength is added or dropped from a wavelength division multiplexed optical signal, thereby making it possible to rapidly change a communication capacity according to a user and constitute a flexible network.
However, in an optical amplifier used in the OADM technology, an arbitrary wavelength is added or dropped, so an input power level (hereinafter, power level is referred to as “level”) to the optical amplifier rapidly changes. For that reason, when the optical amplifier has no control function of following the input level, there arises a problem in that the output level of the optical amplifier transitionally changes.
A description will be given below of a transitional response characteristic in which the output level of an optical amplifier transitionally changes. As indexes indicative of the transitional response characteristic of the optical amplifier, there are a gain excursion, a settling time, and a gain error. FIG. 14 is an explanatory diagram of those indexes which are indicative of the transitional response characteristic of the optical amplifier. The amount of the gain excursion indicates the maximum value of the amount of the gain excursion (or the amount of an output level variation) of the optical amplifier at the time of an input level variation. The settling time is a time to stabilize the gain (or the output level). The gain error indicates the amount of displacement between the gain (or the output level) before the input level variation and the gain (or the output level) after the settling. As described above, a variation in the input level to the optical amplifier leads to occurrence of a phenomenon in which the output level from the optical amplifier transitionally changes.
In association with the above technology, Related Art Document 1 (Japanese Patent Laid-Open No. 2002-290331 A) discloses an example of the optical transmission apparatus to which the OADM technology is applied. In one embodiment of Related Art Document 1, an optical power detecting unit detects the optical power of an optical signal to be outputted from an optical multiplexer and inputted to the optical amplifier. The transmission wavelength property of the optical multiplexer is controlled on the basis of the detected result so that the optical power at each wavelength of the optical signal that is outputted from the multiplexer becomes maximum. Also, in another embodiment, the transmission wavelength of an optical transmitter which becomes an add optical signal is controlled on the basis of the detected result so that the optical power of the optical signal that is transmitted through the optical multiplexer becomes maximum.
In the technology disclosed in Related Art Document 1, because the optical transmission apparatus is controlled so that the optical power of the optical signal that is transmitted through the optical multiplexer becomes maximum, even if there occurs a difference between the wavelength of the optical signal and the transmission wavelength property of an optical multiplexer, control is performed so that the difference is eliminated. As a result, the above technology makes it possible to suppress an increase in coherent crosstalk and a distortion of the signal wavelength which are caused by the wavelength difference, or an increase in beat noise between amplified spontaneous emission (ASE).
However, there arises a problem in that the above control of the transmission wavelength property is incapable of coping with a rapid change in the number of wavelength due to an increase or decrease in the number of signals accompanied by the Add/Drop, or breaking of a line or the like. Hereinafter, an example will be described. When the breaking of a line occurs, there is the possibility that there is a fluctuation of 1 dB or more per 100 msec (millisecond). Also, the amount of level variation becomes 10 dB or more. However, for the control of the transmission wavelength property, in general, a control time takes about several hundred msec, and a control range is equal to or lower than several dB. For that reason, in the control of the transmission wavelength property, when there occurs a rapid fluctuation of the input level due to the breaking of a line or the like, there arises a problem in that the output level of the optical amplifier cannot be suppressed from being transitionally changed.